Former San Antonio Julian Castro is expected to formally launch his bid for the 2020 Democratic presidential nomination with a details-to-come announcement this month.

Democrat Julian Castro talks about exploring a run for president in 2020 at his home in San Antonio, on Tuesday, Dec. 11, 2018. The announcement Wednesday gives the 44-year-old Castro a jump-start on what's likely to be a crowded Democratic primary field that has no clear front-runner. He tells The Associated Press he plans to announce his decision in early January.(Photo: Eric Gay/Associated Press)

Former San Antonio Mayor Julian Castro is expected to formally launch his bid for the 2020 Democratic presidential nomination Jan. 12 in his hometown.

Castro, who also served as secretary of Housing and Urban Development in the Obama administration, stopped short of making his bid official in a media advisory Wednesday, just as he did Dec. 12 when he declared that he was establishing an exploratory committee in advance of a campaign.

However, his twin brother, U.S. Rep. Joaquin Castro of San Antonio, dropped all pretense during a joint appearance on "The Late Show with Stephen Colbert."

"I'll speak on his behalf. He's going to run for president," Joaquin Castro said in response to prodding from the late-night comedian. "For the FEC lawyers — he didn't say it. I said it."

Closer to home, Democratic U.S. Rep. Beto O'Rourke of El Paso has said he hasn't ruled out a run for the presidency. O'Rourke received national acclaim and support during his unsuccessful campaign to unseat Republican U.S. Sen. Ted Cruz.

O'Rourke, who leaves office Thursday, has not laid out a timetable for reaching a decision.

Julian Castro's expected entrance into the 2020 presidential sweepstakes comes just days after Democratic U.S. Sen. Elizabeth Warren of Massachusetts unveiled her own plans for a national campaign. Others, including former Vice President Joe Biden and New Jersey U.S. Sen, Cory Booker, also are said to be considering a run for the Democratic nomination.

U.S. Sen. Elizabeth Warren(Photo: Ryan McBride, AFP/Getty Images)

Julian Castro, 44, has not run for office outside of San Antonio. But in a four-minute YouTube speech last month, he offered a preview of themes he might employ in what could be a long slog for the nomination.

"Growing up in Texas, America wasn't just my home, my country, it was also a promise," he said. "A promise that my family could have a better life. My grandmother, Victoria, came here when she was just 7 years old. She never made it past the fourth grade, but she taught her family the value of hard work."

Since leaving the Obama Cabinet, Castro has established a political action committee called Opportunity First to help elect left-of-center candidates. In October, his memoir, "An Unlikely Journey: Waking Up from My American Dream," was released.

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Juli‡n Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register

Juli‡n Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register

Julián Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register

Juli‡n Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register

Julián Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register

Julián Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register

Juli‡n Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register

Juli‡n Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register

Julián Castro, Former United States Secretary of Housing and Urban Development and Possible 2020 presidential candidate speaks at The Des Moines Register Political Soapbox Aug. 17, 2018.
Zach Boyden-Holmes/The Register